1. Field of the Invention:
The present invention relates to a capacitor element for a solid electrolytic capacitor such as tantalum capacitor or aluminum capacitor. The present invention also relates to a device and a process for making such a capacitor element.
2. Description of the Related Art
As disclosed in U.S. Pat. No. 5,461,538 (corresponding to Japanese Patent Application Laid-open No. 7-74062) and as illustrated in FIG. 10 of the accompanying drawings, a typical capacitor element A for a prior art solid electrolytic capacitor includes a porous capacitor chip A1 and an anode wire A2 projecting from a top face A1a of the chip A1. The capacitor chip A1 is prepared by compacting tantalum powder into a porous mass and then sintering the porous mass. The anode wire A2 is also made of tantalum. For providing a capacitor function, the capacitor element is subjected to the following process steps.
First, as shown in FIG. 11, the porous sintered capacitor chip A1 is immersed in an aqueous solution B of e.g. phosphoric acid and subjected to anodic oxidation (electrolytic oxidation) by applying a direct current. As a result, a dielectric coating A3 of e.g. tantalum pentoxide is formed on the surfaces of the tantalum particles and on an immersed root portion of the anode wire A2, as shown in FIG. 12. In the following description, the portion of the dielectric coating A3 formed on the root portion of the anode wire A2 is designated by reference numeral A3a and referred to as "extension".
Then, as shown in FIG. 13, the dielectrically coated chip A1 is immersed in an aqueous solution C of e.g. manganese nitrate to such an extent that the top surface A1a of the chip A1 is not wet with the manganese nitrate solution, the chip A1 being thereafter taken out of the solution for baking. This step is repeated plural times to form a layer A4 of solid electrolyte (e.g. manganese dioxide) on the dielectric coating A3, as indicated by phantom lines in FIG. 13.
Finally, as shown in FIG. 14, a metallic cathode terminal layer A5 (made of silver or nickel for example) is formed on the solid electrolyte layer with an intervening layer of e.g. graphite being interposed between the cathode terminal layer A5 and the electrolyte layer A4.
The extension A3a of the dielectric coating A3 formed on the root portion of the anode wire A2 is necessary for electrically separating between the solid electrolyte layer A3 (cathode) and the anode wire A2 (anode).
On the other hand, at the time of forming the solid electrolyte layer A4, the capacitor chip A1 is immersed in the manganese nitrate solution C up to the point where the surface C1 of the solution starts bulging at the peripheral edge of the top face A1a of the chip A1 due to the surface tension of the solution. As a result, the top face A1a of the chip A1 is held at a level which is lower by an amount h2 than the surface C1 of the solution. Such immersion makes it possible to maximize the contact area between the dielectric coating A3 and the solid electrolyte layer A4 while preventing the solid electrolyte layer A4 from coming into direct contact with the anode wire A2.
However, if the surface C1 of the manganese nitrate solution C vibrates in the above-described state due to external shocks or forces, the surface tension of the solution is imbalanced, which causes the solution to flow on the top face A1a of the chip toward the anode wire A2 due to the depth h2 of the immersion. As a result, a portion of the solid electrolyte layer A4 may extend beyond the extension A3a of the dielectric coating A3 into direct contact with the anode wire A2, thereby causing shorting between the cathode and the anode.